Field of the Invention
The invention relates to a method for metering a quantity of a reducing agent introduced into an exhaust-gas or exhaust-air stream of a combustion installation. A quantity of reducing agent is introduced into the exhaust-gas. The quantity of reducing agent is set using a functional relationship on a basis of operation-relevant parameters of a combustion installation, an exhaust gas and/or a pollution reducing catalytic converter. The functional relationship is further adapted to a current state of the pollution reducing catalytic converter. In addition, the invention relates to an apparatus for determining and introducing the quantity of reducing agent. German Published, Non-Prosecuted Patent Application DE 43 15 278 A1, correspondingto U.S. Pat. No. 5,628,186, describes a method and an associated apparatus for metering a reducing agent into an exhaust gas of an internal combustion engine containing nitrogen-oxide. The quantity of reducing agent introduced into the exhaust-gas stream is set by a control unit as a function of operation-relevant parameters. If optimized metering of the quantity of the reducing agent is performed, complete catalytic conversion of the nitrogen oxides is achieved by a catalytic converter without using excess reducing agent.
The operation-relevant parameters of an internal combustion engine such as control-rod travel, an air charging pressure or an engine speed are converted into a reducing-agent rate by the control unit with the aid of a predetermined functional relationship in the form of a characteristic map stored in the control unit. The operation-relevant parameters of the catalytic converter such as catalytic activity or storage capacity are likewise stored in the control unit and additionally used to calculate the reducing-agent rate. The aging of the catalytic converter is taken into account by modifying the operation-relevant parameters of the catalytic converter in accordance with aging curves determined in laboratory tests. The aging curves have a direct relationship to the time of operation of the internal combustion engine. A change in the functional relationship of the internal combustion engine due to aging, wear or changed operation conditions is not taken into account by the method presented in the German patent application.
German Patent DE 33 37 793 C2 also describes a method for metering a reducing agent into a nitrogen-oxide-containing exhaust gas of a combustion installation. Aging of the catalytic converter used to break down the nitrogen oxides is compensated for by a corresponding displacement of a stoichiometry curve which describes the relationship between the nitrogen oxide and the reducing-agent concentration.
In the case of both methods, however, a non-optimum quantity of reducing agent is metered in if the characteristics of the internal combustion engine or of the combustion installation including the catalytic converter, which are used to determine the functional relationship between the operation-relevant parameters and the quantity of reducing agent to be metered, change in an unforeseeable manner.